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Nanomaterials 2020, 10, 839; doi:10.3390/nano10050839 www.mdpi.com/journal/nanomaterials
Article
Superior Electrocatalytic Activity of MoS2-Graphene
as Superlattice
Alejandra Rendón-Patiño 1, Antonio Domenech-Carbó 2, Ana Primo 1,*
and Hermenegildo García 1,*
1 Instituto de Tecnología Química (CSIC-UPV) and Department of Chemistry, Consejo Superior de
Investigaciones Científicas-Universitat Politècnica de Valencia, Avenida de los Naranjos s/n,
46022 Valencia, Spain; [email protected] 2 Departamento de Química Analítica, Universitat de Valencia, Av. Del Dr. Moliner s/n, 46100 Burjassot,
Spain; [email protected]
* Correspondence: [email protected] (A.P.); [email protected] (H.G.)
Received: 6 April 2020; Accepted: 21 April 2020; Published: 27 April 2020
Abstract: Evidence by selected area diffraction patterns shows the successful preparation of large
area (cm × cm) MoS2/graphene heterojunctions in coincidence of the MoS2 and graphene hexagons
(superlattice). The electrodes of MoS2/graphene in superlattice configuration show improved
catalytic activity for H2 and O2 evolution with smaller overpotential of +0.34 V for the overall water
splitting when compared with analogous MoS2/graphene heterojunction with random stacking.
Keywords: superlattice; 2d materials; electrocatalytic
1. Introduction
There is considerable interest in developing noble metal-free electrodes that could efficiently
perform water splitting due to the current change from fossil fuels to renewable electricity [1–4]. In
this context, it has been reported that MoS2 could be an efficient electrocatalyst for the oxygen
evolution reaction (OER) replacing IrO2 and Pt [5,6]. MoS2 also shows electrocatalytic activity for
hydrogen evolution reaction (HER) [7–9] and, therefore, it could be an ideal material to perform both
redox processes of overall water splitting.
Regarding the use of MoS2 as electrocatalyst, it has been shown that the assembly of this
chalcogenide with graphene improves its performance as electrodes [10–12]. Graphene introduces
electrical conductivity favouring electron transfer from the catalytic site on MoS2 to the external
circuit. For this reason, there has been considerable interest in developing different procedures for
the preparation of MoS2/graphene heterojunctions to be used in electrolysis [13–15]
In this context, a procedure consisting in the one-step formation of MoS2 strongly interacting
with graphene has been recently reported. The process is based on the pyrolysis of (NH4)2MoS4 and
chitosan as precursors of MoS2 and N-doped defective graphene, respectively [16]. One of the
advantages of this procedure was that chitosan allows for the formation of nanometric films on
arbitrary, non-conductive substrates, such as quartz or ceramics, which, after transformation on
(N)G, becomes electrically conductive and can be used as electrode [17,18]. Also related to the present
study is a recent report from us on the formation of large area boron nitride/graphene heterojunctions
with superlattice configuration [19].
Since both boron nitride and graphene are two-dimensional (2D) materials with very similar
lattice parameters, the term superlattice refers to the coincidence of the hexagons of boron nitride
overlapping those of graphene. Fundamental studies have shown that superlattice configuration of
2D materials heterojunctions can give an assembly with tunable electrical conductivity in contrast to
Nanomaterials 2020, 10, 839 2 of 9
the random heterojunctions of the two materials [20,21]. This modulation of the electron mobility
through the graphene layer by the underlying boron nitride could be reflected in some unique
properties of the superlattice heterojunction, particularly those that are related to electrochemistry
and electrocatalysis [22–24]. In view of these precedents, it would be of interest to determine the
electrocatalytic behaviour of MoS2/graphene heterojunction in superlattice configuration,
particularly when considering the well-known activity of MoS2 for important reactions, such as HER,
OER, and oxygen reduction reaction (ORR) processes [5].
This manuscript reports a novel procedure for the preparation of large area (cm × cm) films of
MoS2/graphene heterojunction in superlattice configuration. It will be shown that this fl-
MoS2/graphene material (fl meaning few layers) exhibits much lower onset potential for HER and
OER than an analogous electrode prepared with random MoS2/G heterojunction, illustrating the
advantages of lattice matching to improve the electrochemical performance.
2. Materials and Methods
2.1. Methods-Exfoliation MoS2 and Preparation of MoS2/fl-G
Molybdenum sulfide exfoliation was carried out while using polystyrene as an exfoliating agent
(ALDRICH). The polystyrene was dissolved in dichloromethane at a concentration of 3 mg/mL and
powdered Molybdenum sulfide was mixed at a concentration of 1.5 mg/mL. The sample was
sonicated while using a Sonic tip (FisherbrandTM Model 705 at 50% 700W for 5 ho per sample with
a on/off sequence consisting in 1 s off and 1 s pulse and an ice bath was used to prevent solvent
evaporation). After sonication, the dispersion was centrifuged at 1500 rpm for 45 min (Hettich
Zentrifugen EBA 21(Hettich, Westphalia, Germany)).
The supernatant was preconcentrated and deposited on quartz films by rotation coating on 2 ×
2 cm2 quartz substrate to prepare the films (APT-POLOS spin-coater(SPS-Europe B.V., Putten, The
Netherlands): 4000 rpm, 30 s). The pyrolysis of polystyrene treatment was performed using an electric
oven and using the following heating program: heating at 5 °C/min at 900 °C for 2 h.
For the preparation of films with random configuration, commercial MoS2 (Aldrich, St. Louis,
Missouri, USA) at a concentration of 1.5 mg/mL and commercial graphene with an area of 700 m2
(STREM CHEMICALS, Newburyport, Massachusetts, USA) at a concentration of 7.5 mg/mL were
added to a 30 mg/mL polystyrene solution (Aldrich, St. Louis, Missouri, USA) in dichloromethane
and the suspension submitted to ultrasounds using a Sonic tip at 700 W for 5 h. After sonication, the
dispersion was centrifuged at 1500 rpm for 46 min. and the supernatant used to prepare films of
randomly configured MoS2/G heterojunction following the same procedure indicated above.
2.2. Methods-Catalytic Measurements
Electrochemical measurements were made in a conventional three electrode electrochemical cell
while using a Pt disc pseudo-reference electrode and a Pt wire auxiliary electrode that accompanies
the graphene modified glassy carbon (GCE) working electrode (BAS, MF 2012, area geometric 0.071
cm2). Voltammetric experiments were carried out with a potentiostatic device CH 920c (Cambria
Scientific, Llwynhendy, Llanelli, Wales, UK), using H2SO4 1.0 M saturated with air as an electrolyte.
The working electrodes were the MoS2/fl-G films that were deposited on graphite. The graphite was
deposited on the quartz films using the automatic carbon coater (sputter coater BALTEC_SCD005
(BAL-TEC AG, Schalksmühle, Germany) with carbon evaporation supplied BALTEC-CEA035).
3. Results
Upper and bottom layers of S atoms sandwiching and internal Mo IV layer constitute the crystal
structure of MoS2. The geometrical arrangement of the three layers is such that top views define
hexagons with alternating edges S and Mo atoms that are located at different planes. These hexagons
are similar in size to those of G and, therefore, a superlattice configuration can be possible for MoS2-
G heterojunctions when there is a coincidence of the two hexagonal arrangements. Figure 1 illustrates
an ideal model of MoS2 structure and graphene layer assembled in the superlattice configuration.
Nanomaterials 2020, 10, 839 3 of 9
Figure 1. Models showing the structures of graphene and MoS2 and their heterojunction in
superlattice configuration.
There are precedents in the literature showing the possibility to obtain MoS2/Graphene
heterojunction with lattice matching geometry [25,26]. It was the leading hypothesis of the present
study that the adaptation of this method could also serve to obtain MoS2/G superlattice since it has
been previously reported the preparation of large area films of BN/G heterojunctions as superlattice
[19].
Scheme 1 illustrates the procedure followed in the present study. As it can be seen there, the
process starts with commercial MoS2 crystals that are subsequently exfoliated in viscous halogenated
solvent while using polystyrene as promoter. After exfoliation, the residual bulk MoS2 crystals can
be removed from the single and few layers MoS2 sheets by decanting the supernatant. Figure 2a
presents AFM (Atomic Force Microscope) images of films of polystyrene containing MoS2. As
expected in view of the plastic characteristic of polystyrene, these films were smooth and have a
thickness about 200 nm (see profile in panel C of Figure 2). The presence of MoS2 in these films was
not apparent from AFM at this moment due to the thickness. In a subsequent step, these films of
polystyrene embedding MoS2 were submitted to pyrolysis in the absence of oxygen at 900 °C. These
conditions have been previously reported to transform polystyrene into thin films of few layers
defective of graphene [27]. One of the main advantages of the present procedure is its reproducibility
and the possibility to prepare large surface areas.
Scheme 1. The preparation procedure of MoS2/G in superlattice configuration. Starting from bulk
MoS2 particles and a solution of polystyrene (Ps) in CH2Cl2. Sonication of the dispersion followed by
sedimentation of bulk MoS2 particles results in a dispersion of exfoliated MoS2 and Ps in CH2Cl2 (i).
Subsequent pyrolysis results in fl-MoS2/G (ii).
Nanomaterials 2020, 10, 839 4 of 9
A considerable shrinkage in the film thickness accompanies this transformation, as it can be seen
in Figure 2b, where films of about 4 nm thickness corresponding to graphene can be seen (Figure 2d).
The frontal image of MoS2/G film shows also the presence of MoS2 particles on this continuous
graphene film. Measurement by AFM of a statistically relevant number of MoS2 particles on graphene
indicate that the lateral size distribution ranges from 20 to 140 nm with an average about 80 nm and
the thickness of the MoS2 is from 2 to 8 nm, with an average about 2. Figure S1 in the supporting
information illustrates the corresponding histograms of the lateral dimension and height. When
considering that the interlayer distance of MoS2 is 0.615 nm, this average thickness corresponds to
less than four MoS2 layers.
Figure 2. AFM image of Ps film on quartz containing MoS2 before (a) and after (b) pyrolysis. The
profiles c and d show the variation in height along the white lines drawn in a (c) and b (d).
The SEM images of these films show a smooth surface with low apparent roughness and without
cracks or pinholes. Figure 3a shows one of these representative SEM images. The TEM of these films
were obtained by scratching a bit of these films. As an example, Figure 3b shows a representative
TEM image of MoS2/G film after detachment from the quartz support.
The composition of MoS2 was confirmed by EDS elemental mapping, determining an overlap of
Mo and S in 1:2 atomic ratio. Figure S2 in the supporting information provides a summary of the Mo,
S, O (from graphene defects), and C elemental distribution for a representative TEM image. These
TEM images show a graphene layer larger than hundreds of nm having dark spots corresponding to
smaller MoS2 particles of size from 20–140 nm. High resolution TEM images show the expected
hexagonal arrangement characteristic of graphene and MoS2 (Figure 3c). Importantly, selected area
electron diffraction at every point of the TEM image shows bright hexagonal spots corresponding to
graphene. Coincident with those spots, there were also other points due to MoS2. The coincidence of
Nanomaterials 2020, 10, 839 5 of 9
the electron diffraction patterns between two 2D materials (Figure 3d) is taken as the best evidence
for the superlattice configuration.
Figure 3. (a) SEM image of a MoS2/G film on quartz. (b) TEM image of a piece of the MoS2/G film
detached from the quartz support. The darker spots in the image correspond to MoS2 particles and
the large, low contrast sheet is the graphene layer. (c) High-resolution TEM image of MoS2/G
visualizing the hexagonal arrangement. (d) Selected area diffraction patterns of MoS2/G sample
showing two sets of coincident spots corresponding to MoS2 and G in superlattice configuration.
It is similarly proposed here that the existing MoS2 sheets resulting from bulk MoS2 exfoliation
are templating during the pyrolysis the formation of the nascent graphene layers in such a way that
the growing graphene is replicating the existing MoS2 sheet resulting in lattice matching throughout
the heterojunction, as in the precedent reporting the BN/G superlattice heterojunction.
XRD and Raman spectroscopy convincingly evidence the presence of both components,
graphene and MoS2, in the heterojunction. Figure S3 in the supporting information shows the XRD
pattern recorded for MoS2/G. This XRD shows a sharp peak at 2θ 14.60 corresponding to the 0.0.2
facet of MoS2. Other peaks of much lesser intensity correspond to other MoS2 planes. In addition to
the peaks of MoS2, a broad diffraction band at 24° due to few layer defective graphene is also recorded
[12].
Nanomaterials 2020, 10, 839 6 of 9
Figure 4 shows the corresponding Raman for the MoS2/G heterojunction. At high wavenumbers,
the expected broad peaks at 2840, 1540, and 1355 cm−1 corresponding to the 2D, G, and D peaks
characteristic of defective graphenes are recorded. Two very sharp lines at 383 and 408 cm−1 that are
characteristic of MoS2 are also seen. No changes in the position of the MoS2 or G bands is observed,
indicating that the superlattice configuration does not alter the lattice constants of MoS2 or G, in
agreement with the lattice match. The difference in wavenumber of these two peaks indicates the
single layer (18 cm−1) or few layers structure of MoS2 (25 cm−1). In the present case, the difference
between the A1g and E2g vibration modes of MoS2 was 24 cm−1, which agrees with the case of few
layers MoS2. This configuration of MoS2 as few layers platelets is also in accordance with the
previously commented AFM measurements of MoS2 particles that were supported on graphene.
Figure 4. Raman spectrum recorded for MoS2/G. The peaks two-dimensional (2D), G, and D marked
on the plot correspond to G, while the sharp intense peaks denoted as 1E2g and A1g are due to MoS2.
Catalytic Activity
The main purpose of this study is to determine the possibility of preparing large area films of
superlattice MoS2/graphene (s-MoS2/G, s- meaning superlattice and corresponding to the material
prepared, according to Scheme 1) heterojunctions that are suitable for electrocatalytic
characterization and compare their electrochemical properties with those of a random
MoS2/graphene (r-MoS2/G, r- meaning random) heterojunction, as stated in the introduction. Aimed
at this purpose, an additional sample of r-MoS2/G was prepared by introducing commercial
preformed graphene during the process of exfoliation of bulk commercially available MoS2 (Aldrich).
Supporting information provides characterization data of r-MoS2/G sample used for the comparison,
including a set of TEM images and EDS analysis with the corresponding elemental mapping. (Figure
S6) Importantly, selected area diffraction patterns show the spots corresponding to hexagonal
patterns of MoS2 and graphene not overlapped and clearly distinguishable for each 2D material, as
expected for samples lacking the superlattice configuration (Figure S5, frame d).
Electrochemical measurements were performed with a conventional three-electrode single cell
with Pt as auxiliary electrode accompanying the MoS2/G electrode and a reference electrode. The
quartz plates used for preparation of electrodes were previously modified by subliming carbon
before MoS2/G film preparation, thus improving the electrical contacts, in order to increase the
Nanomaterials 2020, 10, 839 7 of 9
electrical conductivity of MoS2/G electrodes. Prior controls with sublimed carbon quartz plates
lacking MoS2/G electrocatalyst did not exhibit any peak in cyclic voltammetry. Experiments were
carried out for air-saturated, 1.0 M aqueous H2SO4 solution as electrolyte. Figure 5 shows the cyclic
voltammetry corresponding to s-MoS2/G and r-MoS2/G under these conditions n. As it can be seen
there, for s-MoS2/G a cathodic peak at −0.8 V vs. Ag/AgCl corresponding to ORR is recorded before
a step peak corresponding to HER with onset being determined by extrapolation of the current
density plot at −0.70 V. In the anodic region, the s-MoS2/G electrode exhibits the expected OER process
with an onset potential of 0.87 V. For comparison, Table 1 compiles reported electrochemical data
[28,29] for Pt/C and MoS2 electrodes together with the new data herein obtained for s-MoS2/G.
Figure 5. Cyclic voltammograms measured for s-MoS2/G (a) and r-MoS2/G (b) plotted as
semiderivative of the current density vs. the applied voltage. Conditions: three electrode
configurations; Pt counter electrode; Electrolyte: air-saturated 1.0 M H2SO4. The peaks corresponding
to hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction
reaction (ORR) have been indicated in the plot, as well as the corresponding onset potentials.
Table 1. Electrochemical data for the catalytic HER process of reference electrodes in comparison to
s-MoS2/G. From linear potential scan voltammograms of material-modified graphene modified glassy
carbon (GCE) electrodes in contact with 0.50 M H2SO4; potential scan rate 10 mV s−1. The exchange
current density (jo) was calculated from Tafel graphs using the current extrapolation method.
Material Eonset
(V vs. RHE)
Tafel Slope
(mV per Decade)
Jo
(mA cm−−2)
Pt/C 0.0 30 7.1 × 10−1
MoS2 −0.24 101 9.1 × 10−4
s-MoS2/G −0.60 54 7.9 × 10−3
4. Conclusions
In summary, it has been shown that the exfoliation of bulk MoS2 crystals using polystyrene as
promoter, followed by film casting and pyrolysis is a suitable procedure for obtaining few layers
MoS2 deposited on few layers defective graphene in superlattice configuration. The procedure allows
for obtaining large area films that are suitable for electrochemical characterization. By comparing the
performance of two MoS2/G electrodes, one with lattice matching and another with random
configuration in the heterojunction, it has been observed that the electrocatalytic activity of MoS2/G
improves significantly in the superlattice configuration. This probably reflects the favourable orbital
overlapping and electron migration in the MoS2/G heterojunction when the two lattices match one
on top of the other. The present results show the far-reaching potential of superlattice assembly of
2D heterojunctions for application in electrocatalysis.
Nanomaterials 2020, 10, 839 8 of 9
Supplementary Materials: The following are available online at www.mdpi.com/2079-4991/10/5/839/s1, Figure
S1: Lateral size (a) and height (b) of MoS2 nanoparticles present on MoS2/G, Figure S2: EDX analysis of S, Mo, C
and O for the MoS2/G particles shown in the TEM image (left frame); Figure S3: XRD spectrum of MoS2/G; Figure
S4: Frontal view (a and b) and height profiles (c and d) along the white lines of r-MoS2/polystyrene (a and c) and
r-MoS2/G (b and d); Figure S5: TEM at different magnifications and selected area electron diffraction of r-MoS2/G;
Figure S6: TEM image (left) and EDS analysis of C, S and Mo for r-MoS2/G.
Author Contributions: The research was performed with the contribution of all the authors. The concept of the
study was developed by A.P., and H.G. Sample preparation and characterization was performed by A.R.-P. and
Electrochemical measurements were carried out by A.R.-P and A.D.-C. Drafting of the manuscript was
performed by H.G. and A.P. All the authors corrected and read the manuscript. All authors have read and agreed
to the published version of the manuscript.
Funding: This research was funded by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa
and CTQ2015-68653-CO2-R1) and Generalitat Valenciana (Prometeo 2017-083).
Acknowledgments: A.P. thanks the Spanish Ministry for a Ramón y Cajal research associate contract.
Conflicts of Interest: The authors declare no conflict of interest.
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